In the operation of a self-acting hydrodynamic bearing, a load may be supported by the pressure generated by fluid flow in the bearing gap by the relative motion of the bearing surfaces and viscosity of the fluid. Such bearings may be distinguished according to the composition of the fluid (air vs. oil, grease and the like) and the character of the bearing surface (either plain or grooved), and type of relative motion (journal or thrust).
Bearings which are designed to operate using fluids such as oil and grease require a greater gap than is necessary for an air bearing, the latter having a gap in the range of 2.5 to 5 micrometers. Oil- and grease-lubricated bearings experience a much larger drag torque on the bearing surfaces due to the much higher viscosity of the lubricant, which fills the gap as a thick film. The drag torque can be significant and the bearing starting torque can be undesireably high. The fluid is also subject to loss via leakage or evaporation, and to the detrimental effects of shearing stress and cavitation. As there is a significant risk of contamination of any optical assemblies that are proximate to the bearing, the bearing design must incorporate lubricant seals that are costly and complex, and which are themselves subject to failure.
Air bearings are therefore particularly suited to low-noise, high-speed, high-accuracy bearing applications, which makes them attractive for use in optical scanners In many instances it has been the availability of a gas bearing, which provides a high-accuracy, high-speed spin axis, that has broadened the scope of modern scanning applications. Self-acting air bearings are therefore the predominant bearing design in high-speed rotating polygon and hologon laser deflection systems.
Conventional self-acting air bearings are designed to be entirely free of the oil or grease associated with other hydrodynamic bearings. Unfortunately, a self-acting air bearing requires relative movement of the surfaces to generate pressure in the bearing gap. Some rubbing of the bearing surfaces takes place at start-up (wherein the bearing experiences "lift-off") and shut-down (wherein the bearing experiences "touch-down"). The bearing surfaces must therefore be manufactured from, or coated with, very hard materials such as nickel, ceramics, or cermets. Whereas these materials may be machined to the extremely high surface accuracies required for self-acting gas bearings, such materials are very expensive to fabricate and finish. For example, a ceramic piece may have a unit cost that is ten times the cost of a similarly-shaped piece that is fabricated from stainless steel.
Attempts to reduce starting friction and wear by the use of coatings with high sliding wear resistance, or boundary lubrication with fatty acids, have had limited success. Inserts do not provide the necessary surface accuracy over bearing life that the metallic or ceramic surfaces do. With respect to boundary lubrication, dispersion (creep) of the lubricant remains a problem. The conventional approach seeks to "wet" the bearing surface (i.e. establish a continuous oil film that will separate the bearing surfaces) without causing the lubricant to leave the bearing. Barrier films have been used to control the spread of a lubricant to nonbearing surfaces However, barrier films must be used with great care to avoid contamination of the bearing surfaces which renders them nonwettable and subject to wear.
When rotated, self-acting air bearings are subject to a complex instability phenomena. External vibration at specific frequencies, or poor balance, can stimulate these instabilities. The major factors contributing to instability are typically simplified to include mass/stiffness resonance and high-speed whirl. The former is the well-known type of resonance which occurs in every spring-mass system. However, in self-acting gas bearings, stiffness will change with speed and will not necessarily be linear with deflection. Bearings must be therefore be designed such that this resonance lies outside the frequency range of any likely external vibration and the speed range is below this resonance. If this is not possible, sufficient damping must be built in to limit the vibration amplitude. However, it is difficult to adequately dampen a hologon rotor assembly by means of shock mounting, because the hologon must rotate about its spin axis without the positional deviations associated with the flexure of a shock mount damping apparatus.
Air bearings operate on a very thin air film (5-10 .mu.m) between the bearing surfaces. The thinness and low damping of an air film make it necessary but extremely difficult to analyze the dynamic characteristics of the bearing for compliance, stability, and load carrying capacity at all of the possible excitation frequencies of the system. Nonetheless, the conventional remedy is to maintain close control of manufacturing tolerances, surface finishes, thermal and elastic distortions, alignment, and balancing.
Half-speed whirl is experienced when the average induced flow in a plain journal bearing running with no eccentricity occurs in the same direction as the rotation, but at half the rotational speed If the shaft is deflected from its central position, it will experience a centering force from the wedge effect and a backward torque from shear forces. These forces will cause the shaft to whirl in a direction opposite to the direction of rotation. When this whirl speed reaches the speed of flow around the bearing (i.e., half speed), there will be no relative flow through the gap and therefore the pressure forces will collapse. A bearing operating at twice the speed of its resonant frequency will tend to whirl at its resonant frequency, which is almost certain to cause bearing failure. If the bearing is to exceed this speed, it must be damped.
Fixed-geometry anti-whirl journal bearings have been proposed that achieve some degree of stability margin at the expense of reduced bearing stiffness (i.e., reduced load capacity) or increased friction. These designs include multi-lobes, Rayleigh steps or pockets, and axial grooves. Bearings with a build-in wedge effect, such as those having spiral grooving, are less effected by the instability induced by half-speed whirl, but are not immune to it. Other approaches include the use of attractive- or repulsive-magnet thrust bearings and tilting-pad radial air bearings, which nonetheless have unique drawbacks such as pad flutter, lockup, and pivot fretting.
All of the foregoing remedies increase the cost and complexity of the air bearing Moreover, no conventional approach has resulted in an inexpensive air bearing design that can withstand a series of start/stop cycles of more than about 20,000 cycles.
Accordingly, there remains, a distinct need for inexpensive, self-acting aerodynamic bearing for use in optical applications that can withstand a series of start/stop cycles well in excess of 20,000 cycles.